Bring a large pot of water to a boil. Add the cipolline, boil for 1 minute, then drain. Peel them while they are hot; the papery outer skin should slip off easily. Trim the root end but don't remove it. Cut the onions into narrow wedges with a piece of the root attached so the wedges hold together.

Bring a pot of water to a boil for cooking the peas.

While the water heats, melt the butter in a large skillet over moderately low heat. Add the 6 tablespoons water and the onions and season with salt and pepper. Cook at a gentle bubble until the onions are tender, about 10 minutes; do not allow them to brown. Add the thyme and cook for about 1 minute to release its fragrance.

When the water boils, add the peas and cook until they are just tender, about 3 minutes. Lift them out with a skimmer or slotted spoon and add them to the skillet with the onions. Add the lemon zest and toss well. Taste and adjust the seasoning.

Transfer the vegetables to a serving bowl, then spoon a couple of tablespoons of water from the pea pot into the skillet, swirl to release any flavorful juices, and pour the juices over the peas. Serve at once.